4 BOTANY. Compare this with 39.87 inches, which is the mean precipitation for aseries of years in West Virginia. This State is selected because it has somepoints of similarity to Central Colorado, and because its precipitation is farfrom excessive. The difference is so great as to suggest that this is anessential feature in the difference of the floras in Colorado, where we prob¬ably have a difference as great between the meteorological conditions of itsplains and its mountains, as between West Virginia and Colorado. Another meteorological element will probably be sufficient to explainthe problem in part. Where we have so small a mean precipitation, it issafe to infer that the atmosphere comparatively seldom reaches the point ofsaturation; i. e., that.there is less than the ordinary amount of aqueousvapor in it. Then it follows that however much of the sun's heat beabsorbed by the soil during the day, it will be most freely radiated backinto space at night. I cannot better illustrate the full import of this factthan by a quotation at second hand from Tyndall: 'Aqueous vapor is ablanket more necessary to the vegetable life of England than clothing is toman. Remove for a single summer night the aqueous vapor from the airwhich overspreads this country, and every plant capable of being destroyedby a freezing temperature would perish. The warmth of our fields andgardens would pour itself unrequited into space, and the sun would riseupon an island held fast in the iron grip of frost.' These, then, being the climatic conditions, somewhat, of the plainsduring the growing period of the year, it does not seem strange that theensemble of the flora should be as peculiar. The diurnal range of tempera¬ture during the summer months is at times immense. In South Park, Ihave seen the temperature as high as 90° Fahr. at 2 p. m., and on risingthe next morning found a film of ice coating the little accumulations of wateraround camp. Our familiar forms of plant-life would almost all be destroyedunder such an alternation of heat and frost for year after year. The plants,then, that we do find sui-viving are, as a rule, more dwarfed, more villous,and with denser tissues than those of more genial regions. Nature wouldappear to have especially guarded them against excessive evaporation oftheir fluids on the one hand and freezing on the other, and meeting bothcontingencies by a small supply of water in their tissues, retaining that